An enigmatic creature that lived half a billion years ago is as “strange as life on another planet” and has defied classification for nearly a century. But researchers can finally identify the mystery organism as an animal — the oldest known animal on Earth — thanks to traces of ancient fats.

This precious organic evidence emerged from exceptionally well-preserved fossils found in northwest Russia near the White Sea. The remains come from a strange organism known as Dickinsonia. Dating to around 558 million years ago, Dickinsonia had an oval, segmented body that measured about 5 feet (1.4 meters) long, and it lacked physical features typically associated with animals, such as discernable limbs, orifices or organs, or a discernable head.

For decades, the oddball bodies of Dickinsonia and other peculiar creatures from this period — the Ediacaran, about 635 million to 541 million years ago — made it challenging to place these creatures on the tree of life with certainty. But the recent discovery of additional Dickinsonia fossils revealed something that had never been seen in this type of fossil before: organic tissue preserved in the fossilized impression left behind by the creature’s body. From this impression, or biofilm, researchers were able to identify molecules of cholesterol, a fat that is recognized as “a hallmark” of animals, the scientists reported in a new study.